Application of Surface Spectroscopies and Microscopies to Elucidate
Sorption Mechanisms on Oxide Surfaces
Scott E. Fendorf, Soil Science Division, University of Idaho, Moscow ID 83844
and Donald L. Sparks*, Department of Plant and Soil Sciences, University of
Delaware, Newark DE19717-1303 USA
Abstract
Retention reactions on particulate of soils and waters are of environmental and
agricultural significance because these reactions can greatly affect the risk imposed by
contaminants and the availability of plant nutrients. The surface structure of the sorbate
determines their potential for remobilization to the aqueous environment and the
physical/chemical modifications of the sorbent.
Therefore, it is essential to employ
experimental techniques that provide direct and accurate information on sorption mechanisms.
In this study, the use of multiple surface probing techniques for investigating sorption reactions
is demonstrated for Cr(III) retention on a commonly occurring natural sorbent, amorphous SiC>2
(silica). Extended X-ray absorption fine structure (EXAFS) spectroscopy, diffuse reflectance
infrared Fourier transform spectroscopy (DRIFT), high-resolution transmission electron
microscopy (HRTEM), and surface probing microscopy (SPM) were used to discern the
structural environment of Cr(III) on silica. Chromium(III) formed a monodentate surface
complex on silica, which with increased Cr(III) levels resulted in a nucleated y-CrOOH type
structure. The initial nucleation expanded on the silica surface, but as precipitation progressed
discrete highly-ordered y-CrOOH surface clusters formed rather than a precipitate that was
distributed over the surface. Consequently, when modeling the retention reaction surface
complexation and precipitation must be considered; desorption would be dominated by the
dissolution of the chromium hydroxide surface clusters.
Introduction
Reactions at the solid/solution interface have dramatic influences on environmental
quality and agricultural productivity. Characterization of solids in soils and waters, in addition
to reactions which occur at their surfaces, is necessary to assess the reactivity of a system.
Obtaining direct evidence on interfacial reactions is important to definitively identify reaction
products and the stability of these species. Recent advances in surface characterization
techniques has greatly enhance one's ability to study reactions at the solid/solution interface.
Unfortunately, no single technique provides a complete depiction of most systems. Accordingly,
it is important for researchers to employ multiple techniques to accurately study a system and
thus obtain detailed information of reactions mechanisms.
Surface spectroscopies and microscopies have advanced rapidly and offer a host of
capabilities. For studying retention mechanisms and the properties of" the sorbate and modified
sorbent, methods which yield electronic and structural information on the sorbate and surface
structural (morphological) information on the sorbent/sorbate composite are necessary. To our
knowledge no single technique provides all of this information. Accordingly, it is essential to
employ a multitude of complimentary techniques. The electronic state of sorbed species can be
determined with electronic (UV/VIS) or x-ray absorption spectroscopies. In some cases,
^canning tunneling microscopy (STM) can also be used to derive electronic states of surfaces.
182
Magnetic, vibrational, and x-ray absorption spectroscopies can be used to determine the local
structure of a sorbate.
Microscopic techniques, such as electron and scanning probe
microscopies, are needed to discern the structure and morphology of the sorbent/sorbate surface.
A powerful means for studying the local chemical and structural environment of an
element in solutions, suspensions, or solids, is x-ray absorption fine structure (XAFS)
spectroscopy. XAFS provides one with information on the oxidation state of the x-ray absorbing
element along with its local structure on a very quantitative basis. While atomic information is
probed with XAFS, Fourier transform infrared (FTIR) spectroscopy can be employed to ascertain
molecular interactions of a sample. Often XAFS can be used to give precise structural
information as to the source of FTIR spectral features. Neither of these spectroscopies, however,
provide spatial resolution unless the probed element or molecule is limited to a certain region of
the sample—which is the case for most spectroscopies except, for example, Auger spectroscopy.
Microscopic techniques thus compliment spectroscopic studies since they provide spatial
information, often which can be obtained with atomic resolution. They do not, however, provide
information exclusive to a single element. Transmission electron microscopy (TEM) can be
employed to discern the atomic ordering of a sample along with surface modifications.
However, surface modifications must possess a depth dimension sufficient to attenuate the
impinging electrons. This restriction can be circumvented with scanning probe microscopes
(SPM), such as atomic force (AFM), which provides detailed atomic and morphological
information that is restricted to the surface. Additionally, SPM can be conducted in aqueous
systems, thus removing the need to subject the sample to an altered analytical environment. In
this study, the utility of a composite surface analysis using XAFS, FTIR, TEM, and SPM will be
exemplified with Cr(III) sorption on silica.
Inorganic compounds are potential pollutants which can be particularly problematic due
to their stability in the environment. Chromium is an environmentally significant metal used in
various industrial processes. Of the two Cr oxidation states that are stable in the environment,
Cr(VI) is much more hazardous than Cr(III) and is very mobile in soils and waters. Oxidation of
Cr(III) by manganese oxides makes this rather benign species a potential hazard. One possible
mechanism for retarding Cr(III) oxidation is its retention by other non-redox reactive sorbents
present in soils or waters. This would 'prevent oxidation by inhibiting Cr(III) from complexing
with manganese oxides.
Although commonly only a small fraction of the solids in soils or sediments, hydrous
oxides (i.e., hydroxides, oxyhydroxides, and oxides) can play an important factor in the
environmental behavior of metal ions. The influence of hydrous oxides is enhanced by the
formation of high surface area particles and surface coatings on other soil materials. Silica
(Si02) is a commonly occurring hydrous oxide which readily forms coatings on other solids. It
often has a very high surface area and thus exerts a large influence on sorption reactions.
Moreover, sorption reactions are extremely influential on the risk of metal ions in the
environment since the metals are removed from the mobile aqueous phase. Hence, to determine
the hazard of Cr in the environment, knowledge of Cr(III) sorption on non-redox reactive solids,
such as silica, is necessary.
Determining surface structures is necessary to elucidate reaction mechanisms, the
stability of bound species, and for evaluating the physical/chemical properties of the modified
sorbent. Therefore, the objective of this study was to investigate the surface structure of Cr(III)
sorbed on Si02 and to determine the surface modification and stability of the bound phase. By
employing XAFS, DRIFT, SPM, and HRTEM a composite of information was obtained, which
allowed for an accurate and detailed analysis of the Cr(III)-Si02 sorption mechanism. The local
chemical and structural environment of Cr(III) was ascertained with XAFS, molecular
information with DRIFT, and HRTEM and SPM provided spatial resolution and information on
the surface structural modification of the silica after reaction with Cr(III). The DRIFT spectra
183
were evaluated by comparing vibration modes of 'neat' silica and y-CrOOH to the Cr-silica
systems. The results gleaned with XAFS were compared to the information obtained with
DRIFT analysis. This permitted the use of DRIFT for evaluating the effect of an extensive range
of reaction parameters on the Cr(III) surface structure. Atomic-ordering and surface alterations
induced by Cr(III) sorption on the silica surface were ascertained with HRTEM and SPM.
Materials and Methods
Batch Studies
The silica used in this study was a Huber Zeothix® 265 amorphous SiC*2 colloid,
synthesized as described by Wason (1978). The oxide was washed in pH 3.5 HNO3 and then
dialyzed in doubly distilled deionized water until a stable conductivity resulted for 24 h. The
surface area was 221 m2 g-i, as determined by the ethylene glycol monoethyl ether (EGME)
method (Heilman, 1965). The particle size of the oxide was less than 2.0 |im.
Batch studies were performed to determine the amount of Cr(III) sorbed on SiÜ2 as a
function of pH and initial Cr(III) concentration ([Cr]0). A pH range of 3 to 7 was investigated
with initial concentrations of 100, 200,400, and 5 x 103 \iM Cr(III). For the batch studies, 0.5 g
SiC"2 was dispensed in reaction vessels with 2 L of 0.1 M NaNC>3. A 10 mM Cr(III) stock
solution was used to obtain the desired Cr(III) concentrations. The stock solutions were made
from ACS reagent grade Cr(NÜ3)3 • 9H2O, with acidified deionized water (pH < 2) and were
never allowed to age more than five days to limit potential polymerization.
The oxide was allowed to hydrate for 48 h prior to reaction. After the hydration period
the pH was adjusted, the desired amount of Cr(III) was added, and the final volume brought to 2
L, yielding a suspension density of 0.25 g L-i. The pH was held constant with a pH-stat system;
upon reaching a steady pH the vessels were placed in a water bath reciprocating shaker. After 48
h the samples were filtered through a 0.22 urn pore membrane and the solution analyzed for Cr
with a JY-70 ICP spectrophotometer. The sorption data and structural information gleaned in
this study are representative of the reaction after a 48 h time period, and therefore may not be
indicative of sorption at equilibrium (Wehrli et al., 1990).
All reactions were carried out at 25 ± 0.5 °C at 1 atm pressure in a N2 (g) environment to
eliminate the influences of CO2. The initial solutions were purged with N2 (g) prior to the
addition of the oxide. Thereafter, a N2 (g) stream was maintained over the surface of the fluid
when the vessels were exposed to the surrounding environment.
In this study the data are referenced to the potential surface site occupancy, <|),
<>
| = (mols Cr sorbed)/(mols surface sites)
[1]
One should be aware that this does not imply a site occupancy but only a potential maximum if
each sorbed Cr(III) occupied a single site. If polymerization or nucleation occurs then a different
amount of the surface sites would be occupied (i.e., an amount less than § would be covered).
This parameter, <j>, is employed because it allows one to easily recognize the quantity of sorbed
Cr necessary for monolayer coverage; at less than potential monolayer coverage isolated site
binding can account for the total sorption quantity, but at $ > 1 nucleation or multi-layer
adsorption is required. An estimated reactive surface site density of 5.5 sites nm 2 (Fouad et
al., 1991) was used to calculate the site density for silica. For XAFS studies, samples reacted at
184
pH 6 were employed for analysis because this pH is common to soils and waters, and represents
complete uptake of aqueous Cr onto the silica surface.
Solids were consolidated prior to their analysis. The equilibrated suspensions were
passed through a 45 mm diameter 0.22 ^m membrane filter; the filtrate was then rinsed with 100
ml of high purity water to remove the entrained electrolyte. The solid material was then placed
in a vial and sealed. The samples remained moist and were never allowed to dry. In addition, a
hydrous chromium oxide (HCO) was precipitated by titrating 20 mM Cr(III) to pH 6, with pH
maintained at this level for 24 h. This procedure was similar to that used by Charlet and
Manceau (1992).
XAFS Studies
XAFS spectroscopy was performed at the National Synchrotron Light Source,
Brookhaven National Laboratory, under dedicated running conditions on beam line X-l 1A. A
Si(lll) water cooled double crystal monochromator was employed with a sagitally focused
beam (Lamble and Heald, 1991). The focused beam provided an intensity greater than three
times that provided by the conventional flat monochromator arrangement over the energy range
used for this study. A 0.5 mm pre-monochromator slit width was employed, which was
readjusted as necessary to compensate for vertical motion of the stored electron beam. Higherorder harmonics were rejected by detuning 30% from the maximum incident intensity (I0).
Reference materials were run in transmission mode and the transmitted intensity
measured with an ionization chamber. All Cr(III)-Si02 systems were analyzed by fluorescence
detection. Samples were analyzed at ambient and N2 (i) temperatures (298 and 77 K); no
structural differences were discerned between these temperatures and consequently the 77 K
spectra are reported due to the reduction in thermal disorder.
All samples were run at least in triplicate. Data analyses were accomplished by
optimizing the fit of predicted spectra with Fourier filtered experimental spectra. Phase shifts for
O, Si, and Cr were obtained using model compounds: Cr using a Cr-metal and <x-Cr203, O with
a-Cr2C»3, and Si with NiSi. Amplitude parameters were defined based on a-Cr2Ü3. Successive
shells were isolated in the Fourier transformed spectra, back-transformed, and the interatomic
distances (r's), coordination numbers (CN's), and Debye-Waller factors (2a2) varied until the best
fit was obtained between the predicted and experimental curves. After obtaining the structural
information for each successive shell out to 4 A , the structural parameters were then combined,
to model the entire spectra. The resulting r values for the first coordination shell were accurate
to ± 0.03 A and CN to ±20% based on fits to model compounds-more distant shells will.be
successively less accurate in both r and CN. Further details on the XAFS analysis are given
elsewhere (Fendorf et al., 1993).
Infrared Analysis
Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy was performed
using a Perkin-Elmer FT-IR 1720X spectrometer with a diffuse reflectance accessory. Samples
were prepared using the batch methods described earlier; after reaction the solid material was
washed with 100 ml of deionized water and the solids dried at 60°C for 24 h. The dried material
was then diluted by 90% (by weight) with KBr to reduce the influences of spectral reflectance.
Reported spectra are the average of 200 scans.
185
In addition to the Cr-Si02 samples, reference materials were analyzed for comparison.
Reference spectra were obtained for HCO (the y-CrOOH type structure) and SiC>2. EXAFS
analysis and electron diffraction patterns confirmed that the precipitated HCO material was yCrOOH.
The standards were also diluted by 90% (by weight) with KBr prior to DRIFT
analysis.
Microscopic Analysis
High-resolution TEM was performed on high surface coverage Cr(III) reacted silica,ty=
9.9. For HRTEM analysis, 0.25 mL of the reacted suspension was dispersed on a holey carbon
film supported by a fine mesh copper grid. The oxide coated grids were then rinsed with 50 mL
of deionized water, and dried in a glass chamber. Imaging was then performed on a Hitachi
9000NAR electron microscope. Scanning probe microscopy was performed on moist samples
after filtration on a Digital Instruments NANOSCOPE II operating under conventional force. A
low pass filtering (not Fourier filtering) of the acquired images was conducted to improve image
quality.
Results
Local Structure ofCr(III) on Silica
The extended portion of the XAFS (EXAFS) spectra was utilized to discern the local
structural environment of Cr(III) sorbed on silica. A range of initial Cr(III) concentrations (100,
200, 400, and 5 x 103 \LM ) at pH 6 were investigated with 0.500 g SiC<2 present. Batch
adsorption isotherms revealed that all of the aqueous Cr(III) at pH 6 was depleted. Thus, these
initial Cr(III) concentrations correspond to potential site occupancies (<|>) of 0.20, 0.40, 0.79, and
9.9. The experimental spectra for the Cr-Si02 systems are shown in Fig. 1 along with that of
HCO, Cr(OH)3 • nH 2 0.
The structural parameters derived from EXAFS analysis are summarized in Table 1. The
highest surface loading (<j> = 9.9) is not reported due to possible influences from self-absorption
processes arising in these more concentrated samples. In all of the Cr specimens approximately
six O were observed at 1.99 A. In y-CrOOH, a second shell composed of two Cr at 2.99 A (edge
sharing CrO<s octahedra) and a third shell of 1.5 Cr at 3.98 A (corner sharing octahedra) were
discerned. These results are in good agreement with the structural parameters determined by
others for y-CrOOH (Manceau and Charlet, 1992; Corker et al., 1991). The first two shells (i.e.,
six O at 1.99 A, and two Cr at 2.99 A) in all the Cr(III)-Si02 samples were similar to those of the
Cr-hydroxide, with slight variations in the CN. However, a different third shell resided in these
materials: a single Si atom at 3.39 A (Fig. 2). Consolidating the structural parameters derived
from the isolated shells resulted in good agreement between the predicted and experimental
EXAFS functions for the samples (Fig. 3).
The Si-Cr distance of 3.39 A represents a monodentate complex, in agreement with the
CN, and would necessitate a 150° Cr-O-Si bond angle. Beyond the Si shell another Cr shell
occurred; however, the EXAFS intensity of this shell is much weaker than the inner-shells and
therefore one must place less confidence in the derived structural values. Nevertheless, further
structural information can be obtain by considering the crystallographic parameters of the sorbent
and sorbate. Assuming that the silica surface is dominated by alternating tetrahedra, then Cr(III)
186
^
^
-1
Figure 1. Experimental EXAFS spectra of Cr(III) sorbed on silica.
187
Table 1. Structural information derived from EXAFS analysis: interatomic distances (r, A),
coordination numbers (CN), and the Debye-Waller factors (2CT2, A 2 ). The O, edge sharing Cr
octahedra, and Si coordination shell of a central Cr are reported.3
Cr-O
r(A) CN 2a 2
Edge: Cr-Cr
r(A) CN 2o 2
r(A)
Si-Cr
CN
y-CrOOH
1.99 5.7 0.008
2.99 1.5 0.006
<|>=0.79b
1.99 6.2 0.005
3.00 2.3 0.010
3.38
1.1
0.009
()>=0.40b
1.99 6.0 0.005
2.98 2.5 0.010
3.40
1.0
0.008
<|>=0.20b
1.98 6.2 0.005
3.01 2.9 0.011
3.39
0.71 0.009
2o 2
a
Based on standards and internal deviation, the reported values are accurate to within r ± 0.03 A., CN ± 20%.
"Potential surface site coverage for Cr(III) sorbed on silica (mols Cr / mols surface sites).
in a monodentate complex with a bond angle of 150° would result in a favorable distance,
approximately 3.9 A, for hydroxyl bridging between adjacent Cr(III). Based on this premise, the
resulting surface structure is a monodentate surface Cr(III) complex and a HCO precipitate with
the y-CrOOH type structure that is shown in Fig. 4.
DRIFT Analysis
The DRIFT spectrum for y-CrOOH is shown in Fig. 6. One notes a strong, sharp
absorbance at 1385 cm 1 and a broader one at 1417 cm 1 . These peaks are characteristic of the
bending modes for metal oxyhydroxides (Me-OH-Me) with the boehmite type structure (yMeOOH) (Ryskin, 1974) and correlate well with the previously defined deformation modes of yCrOOH (Snyder and Ibers, 1962). The 1385 cnr 1 peak is thus assigned to the in-plane (y-OH)
bending mode and the 1417 cm-1 to the out-of-plane (8-OH) mode. The presence of these modes
in the Cr-Si02 systems may be evidence for the formation of a y-CrOOH precipitate. The DRIFT
spectrum of unreacted SiC"2 is also given in Fig. 7. The 'neat' silica has a strong characteristic
water vibrational band at 1630 cm-i. However, the absorbance in the range of 1350-1600 cm-i is
minimal, and thus does not interfere with the prominent bending modes of y-CrOOH.
Consequently, it appears that the 1385 and 1417 cm-' bands in the Cr-Si02 spectra should be
good evidence for the formation of a y-CrOOH surface precipitate, while their absence would
guggest that surface nucleation of this phase did not occur.
188
—
experimental
predicted
^
^
8
10
12
-I14
Figure 2. The Fourier back-transformed EXAFS function for the isolated second peak of the Fourier transformed
spectra. The predicted fit was obtained by incorporating Si and Cr shells.
A third spectrum is shown in Fig. 7: Cr(III) sorbed on silica (<)> = 9.9). In this spectrum
1385 and 1417 cm-1 peaks are visible but additional absorbances at 1450 and 1550 cm-1 are also
apparent. The 1385 and 1417 c m 1 absorbances are identical to those observed for the
deformation modes in y-CrOOH and therefore represent the formation of a surface precipitate.
The 1450 and 1550 c m 1 peaks are not found in the Cr-hydroxide spectra; their appearance and
magnitude are correlated except under conditions where the 1450 cm 1 peak is obstructed by the
1417 cnr 1 absorbance. These peaks may be representative of a Cr-O-Si interaction. The 5-0
and y-0 bending modes in a monodentate Cr-O-Si bond should be shifted to higher energies than
for Cr-OH-Cr (Ryskin, 1974). Consequently, the 1550 cnr 1 peak is assigned to the S-O and the
1450 cnr 1 to the y - 0 vibration in a Cr-O-Si linkage. Table 2 presents the assignment of the
peaks for the DRIFT spectra of these materials. With the assignment of the DRIFT peaks in the
1350 to 1600 cm -1 region one can identify structural factors arising from the sorption of Cr(III)
on silica. In addition, the limits of DRIFT for detecting the surface phases in these systems can
be determined.
The DRIFT spectra of 40,100, 200, 400, and 5 x 103 \)M [Cr]0, resulting in <s> values of
0.079, 0.20, 0.40, and 9.9, are presented in Fig. 7. The latter four conditions were identical to
those used in the XAFS study permitting a comparison of results from these two techniques.
With <>| = 0.040 no alteration from the unreacted Si02 spectrum was observed. As § increased
189
Figure 3. The EXAFS function for the Fourier back transformed spectra. The theoretical line was derived with
parameters obtained from analysis of the isolated shells.
190
experimental
predicted
Cr(OH),3 • "nH
0
" 29'
H
PH
(j) = 0.79
({) - 0.40
$ = 0.20
-1
Figure 4. Fourier transformed spectra resulting in the radial structure function (RSF) of the inner 4 A shells: HCO
and Cr sorbed on silica are shown. The first peak results from six O at 1.99 A; the second from Cr at
2.99 A . In the presence of Si, the second peak incorporates Si at 3.39 A.
191
Cr
^ V
O
\
Cr- 2.99A-^Cr
/
OH , '
3.97 A
;Cr
°
,*'
OH
OH
Figure 5. A depiction of the surface structure derived by EXAFS analysis (a) showing the interatomic distances for
Cr sorbed on silica. The y-CrOOH type structure is shown forming in (a) and in (b) the interatomic
distances of this phase are given.
to 0.079 relatively broad bands become apparent at 1450 and 1550 cm -1 . These broad bands
increase in magnitude with further increased <>
t and are the most prominent at the highest
coverage (<> = 9.9) depicted in Fig. 7. A sharp peak at 1385 cm-1 and a broader band at 1417
cm-i are apparent at <>| = 0.20 (100 yM [Cr]0). The four peaks at 1385, 1417, 1450, and 1550
cm-1 all increase with continued increases in surface coverage, but the 1385 and 1417 cm 1 peaks
clearly begin to dominate this spectral region at higher surface coverage. At <>| = 9.9, the 1450
cm-1 becomes only a poorly resolved shoulder on the 1417 cm-1 absorbance.
192
Silica
Cr-OH-Cr
1417
^
\
1385
5-OH Y-OH
Y-CrOOH
1550
Cr-O-Si
1450
S-OH
Cr(III)-Si02
T
1650
i
Y-OH
^
i
1550
1450
-1
1
1350
cm
Figure 6. The DRIFT spectra for silica, Cr(OH)3 • nH20, and for Cr(IIl) sorbed on silica with 0 = 9.9. The peaks at
1385 and 1417 cm"1 arise from the Cr-OH-Cr deformations, and the 1450 and 1550 cm"1 peaks are due
toCr-O-Si deformation.
At <() = 0.040 (20 \iM [Cr]0), no structural modifications are apparent with DRIFT
spectra. However with a two-fold increase in coverage, 0.079, the monodentate surface complex
(Cr-O-Si) was observed. Hence the detection for sorbed Cr on silica in this study was between
0.04 and 0.08 surface coverage. The 0.079 coverage spectra do not indicate the presence of
nucleated Cr-hydroxide. Nucleation of y-CrOOH is discerned at 0.20 coverage as noted by the
1385 and 1417 c m ' absorbances. Hence, DRIFT spectra indicate that a monodentate surface
complexation of Cr on silica occurs at 0 < 0.20 (100 [LM [Cr]0), and y-CrOOH nucleation
appears at 0 ^ 0.20.
193
Table 2. The assignment of the deformation modes for y-CrOOH and surface complexed Cr(III)
on silica.
frequency (cm - 1 )
bond type
mode
1385
Cr-OH-Cr
Y-OH
1417
Cr-OH-Cr
6-OH
1450
Cr-O-Si
y-O
1550
Cr-O-Si
8-0
O
u
o
1650
1550
1450 1417 1385 1350
cm •1
Figure 7. DRIFT spectra for Cr(III) sorbed ort silica at [Cr] 0 of: 40 \iM (d> = 0.079), 100 uM (4i = 0.20), 200 \iM
(<(> = 0.40), and 400 uA/ (4> = 0.79). The peaks at 1385 and 1417 cm" 1 represent Cr-hydroxide
nucleation, while the absorbances at 1450 and 1550 cm"1 indicate a monodentate Cr-SiC>2 complex.
194
Microscopic Surface Analysis
Although both EXAFS and DRIFT give important information on the local structural
environment of Cr(III), it is difficult to determine the spatial proximity and long-range order of
the sorbed material with these two methods. Accordingly, HRTEM and SPM were employed to
deduce further structural information.
Imaging was performed prior to and after reaction with Cr(III) at ty = 9.9 (Fig. 8). The
silica was completely amorphous showing no indications of ordering (marked A). After reaction
with Cr(III) distinct crystalline areas approximately 20 nm x 20 nm are clearly visible (marked
B). The crystalline areas are a y-CrOOH surface precipitate. The HRTEM images reveal that
even when there was greater than ten times the amount of Cr necessary to occupy all the reactive
surface sites discrete crystalline particles formed. This finding indicates that Cr-hydroxide
nucleation does not progress over the silica surface but rather formed isolated nucleated areas
which, at least with high potential surface coverage, possess long-range order (crystallinity).
Figure 8. High-resolution TEM image of silica after reaction with 5 mM Cr(III) at pH 6 (<|> = 10). A discrete
crystalline y-CrOOH surface precipitate (marked B) has formed on the amorphous silica (marked A).
To further explore the surface distribution and surface morphology of Cr(III) on silica we
employed a surface discriminating microscopy: scanning probe microscopy (SPM). A SPM
image of the unreacted silica is shown in Fig. 9a. After reacting with Cr(III) a dramatic surface
^alteration is observed (Fig. 9b). At potential surface coverages ten times monolayer coverage,
195
discrete island structures are discerned on the silica surface. The SPM images confirm the
observations made with HRTEM: the HCO surface precipitate does not distribute over the
surface, but rather nucleates forming surface clusters which grow out from the surface. The
reacted colloid thus maintains areas of unreacted exposed silica with island structures of HCO
protruding from the substrate material. At <>| = 9.9, the clusters average about 60 nm in height
and 35 nm in diameter—as revealed with SPM. These findings have important implications on
the stability of the bound Cr(III) and resulting reactivity of the conglomerated colloid.
Discussion
Both EXAFS and DRIFT analysis indicate that Cr(III) formed an inner-sphere
monodentate surface complex on silica. A Si-Cr distance of 3.39 A was determined; a linear
Cr-O-Si arrangement would result in a distance of 3.59 A, thus a bond angle of 150° is necessary
to produce the observed distance. This Cr-Si distance and angle are similar to that observed for
Mn in MnSi03 (Wyckoff, 1963). Monodentate coordination with a bond angle of 150° implies
that the reactive surface O atoms are singly coordinated by Si. This complexation structure
agrees with the multisite surface complexation (MUSIC) model (Hiemstra et al., 1989a,b) which
indicates that only the singly coordinated O of Si02 are reactive in the pH range of 2 to 10.
McBride et al. (1984) and McBride (1982) also observed that Cu 2+ sorbed only to singly
coordinated 0(H) groups on Al-oxides.
At potential site occupancies (<|>) below 0.20, only Cr(III) complexed with silica was
observed. At coverages equal to and exceeding 0.20, surface nucleation of the y-CrOOH type
structure occurred. A progression from isolated site binding at low coverages to surface
hydroxide nucleation of Cu2+ on Al-oxides was similarly observed (McBride et al., 1984;
McBride, 1982). When nucleation of adjacent surface bound Cr(III) species occurs, HCO
nucleation results.
Chromium(III) sorption on a-FeOOH results in a surface precipitate with the a-CrOOH
type structure (Charlet and Manceau, 1992); as surface coverage increased nucleation growth
expanded over the surface before expanding outward (away from the surface). Further increases
in Cr(III) levels resulted in nucleation progressing outward from the a-FeOOH surface and there
was a phase transition to the y-CrGOH type structure. Our results indicate different phenomena
for Cr(III) sorption on silica. A monodentate complex of Cr(III) on silica forms; the
complexation continues but nucleation of a y-CrOOH type phase occurs well before monolayer
coverage. As nucleation progresses, discrete surface clusters, an island structure, forms rather
than distributing over the surface.
It appears that it is more energetically favorable for surface hydroxide nucleation than for
complexation of Cr(III) on silica at surface coverages of only 0.20. Furthermore, the precipitate
does not grow across the silica surface, but instead forms crystalline clustered structures which
protrude away from the surface. Therefore, the sorbent imparts an important influence on
surface precipitates. On a surface such as a-FeOOH where the structural environment is similar
to that for chromium hydroxides, nucleation progresses over the surface—epitaxial growth. Once
a monolayer has been exceeded nucleation progresses outward and some distance from the
goethite surface undergoes a phase transformation to adopt a more favorable structure (i.e., a
phase transition from a- to y-CrOOH). A different scenario, however, arises with Cr(III)
sorption on silica. Chromium(III) binds to the surface initially in an isolated site mechanism.
Increased retention bridges the Cr species, thus beginning nucleation. In response to the
energetics of an unfavorable surface O-O distance on silica, nucleation of HCO begins to
progress outward from the surface rather than distributing across it. Even at 10 times monolayer
1%
Figure 9. Scanning probe micrograph of (a) silica prior to and (b) after reacting with Cr(III) at potential surface
coverages ten times monolayer capacity. The reacted images (b) confirm that the HCO precipitate does not
distribute over the surface, but rather forms discrete surface clusters which result in the island structured image.
Even at this high surface loading of Cr(IH) many areas of the substrate, silica, remain essentially unaltered.
197
coverage discrete surface clusters are observed with regions of the silica unaltered by the
reaction.
Conclusions
Although important information was gleaned with each of the techniques employed, a
complete depiction of the Cr(III) sorption mechanism was only discerned by considering the
composite of surface information obtained. In order to accurately resolve sorption mechanisms
researchers should therefore consider using a multitude of methods that glean atomic information
and which support and expand upon each others information. In this study, EXAFS and FTIR
spectroscopies were used to obtain molecular information on sorbed Cr(III) while electron and
scanning probe microscopies provided surface morphologies and structural alterations.
The results on this study can be summarized as follows: (1) Aqueous Cr(III) is depleted
by sorption on silica at pH 6, (2) A monodentate Cr(ffl)-SiC>2 surface complex forms under these
reaction conditions, (3) Chromium hydroxide (HCO) surface nucleation results at < 0.20 surface
coverage; the local structure of the sorbate is of the y-CrOOH type structure, (4) Nucleation of
HCO does not progress over the surface but rather forms discrete highly crystalline surface
clusters on silica.
Important conclusions relevant to environmental quality and agricultural production are
thus gleaned from this information. One must consider monodentate surface complexation and
surface precipitation when predicting Cr(III) retention on silica. Additionally, the potential for
desorption of Cr(III) will be dominated by the dissolution of the y-CrOOH islands. And finally,
the reacted'conglomerated colloid will have physical and chemical properties of both unreacted
silica and HCO; future reactions will thus be effected by the properties of these two phases.
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